Seasonal changes in plasma levels of thyroid hormones and the effects of the hormones on cellular ATP content in common bottlenose dolphin

Circulating T4 levels in autumn and winter were lower than those in spring and summer in dolphins in Taiji where WT and AT varied greatly from season to season. T4 levels showed a small difference between spring and autumn in dolphins in Okinawa with warmer WT and AT in smaller amplitude ranges than in Taiji. Total T3 level in Taiji was highest in spring and lowest in autumn as T4 levels, but not significant. The BT of dolphins in Taiji was also lower in autumn and winter compared with those in spring and summer, whereas the BT of dolphins in Okinawa fell in autumn but rose in summer, albeit to a lesser extent than in Taiji. Cellular ATP was increased by administration of both T3 and T4 compared to control. Collectively, these results suggest that the cellular metabolic activities regulated by THs may be enhanced in dolphins exposed to increasing surrounding temperature for lipolysis and reduced in dolphins exposed to colder conditions for fat accumulation.